This invention relates to apparatus for radiation absorption measurement and, more particularly, to apparatus for determining the moisture content of a moving web of material by measuring the relative reflectance of two beams of infrared radiation.
A well-known method for determining the content of a substance such as water in a material involves the measurement of relative infrared reflectance. At certain characteristic wavelengths corresponding to resonance of a particular substance, the absorption, and thus reflectance, of the material being analyzed varies considerably with the content of the substance, while at other wavelengths not coinciding with a resonant wavelength, the degree of absorption is relatively insensitive to changes in material composition. By measuring the ratio of the reflectance of the material at resonant and nonresonant wavelengths, the content of the substance can be determined simply and rapidly. In many practical applications of this technique, both the resonant-wavelength beam and the nonresonant-wavelength or reference beam are derived from a single radiation source using a chopping wheel to obtain alternating pulses of radiation. One such implementation is shown in U.S. Pat. No. 3,150,264, issued to R. C. Ehlert.
Systems of the type described above are often used in on-line applications to measure the moisture content of webs of paper pulp or the like. Such systems, however, are susceptible to errors resulting from the nonlinearity inherent in any practical detector used to sense infrared radiation. Because of this nonlinearity, the reflectance measurement is sensitive to spurious sources of infrared radiation such as the moving web itself, which may be as hot as 300.degree. F. Variations in the absolute amplitudes of the reflected resonant and nonresonant radiation pulses due to sheet flutter and the like will also affect the measurement, even though the amplitudes of the respective pulses vary proportionately. Finally, the nonlinearity of the detector makes the reflective ratio measurement sensitive to changes in operating point due to changes in the ambient temperature.
Error may also result when the sheet flutter contains frequency components near the chopping frequency, which is typically about 10 Hz. Since the apparatus cannot distinguish between amplitude variations due to sheet flutter and those due to changes in material composition, the measured reflectance ratio will contain a spurious component at a frequency equal to the difference between the flutter frequency and the chopping frequency.